Mineral oil based lubricants are conventionally produced by a separative sequence carried out in the petroleum refinery which comprises fractionation of a paraffinic crude under atmospheric pressure followed by fractionation under vacuum to produce distillate fractions (neutral oils) and a residual fraction which, after deasphalting and severe solvent treatment may also be used as a lubricant base stock usually referred as a bright stock. Neutral oils, after solvent extraction to remove low viscosity index (V.I.) components are conventionally subjected to dewaxing, either by solvent or catalytic dewaxing processes, to the desired pour point, after which the dewaxed lube stock may be hydrofinished to improve stability and remove color bodies. This conventional technique relies upon the selection and use of crude stocks, usually of a paraffinic character, which produce the desired lube fractions of the desired qualities in adequate amounts.
The range of permissible crude sources for lubricant production may be extended by the lube hydrocracking process which is capable of utilizing crude stocks of marginal or poor quality, usually with a higher aromatic content than the best paraffinic crudes. The lube hydrocracking process, which is well established in the petroleum refining industry, generally comprises an initial hydrocracking step carried out under high pressure in the presence of a bifunctional catalyst which effects partial saturation and ring opening of the aromatic components which are present in the feed. The hydrocracked product is then subjected to dewaxing in order to reach the target pour point since the products from the initial hydrocracking step which are paraffinic in character include components with a relatively high pour point which need to be removed in the dewaxing step.
Mineral oil derived lubricants have been severely constrained to match the lubrication demands of issuing from modern automotive engine development. Current trends in the design of automotive engines are associated with higher operating temperatures as the efficiency of the engines increases and these higher operating temperatures require successively higher quality lubricants. One of the requirements is for higher viscosity indices (V.I.) in order to reduce the effects of the higher operating temperatures on the viscosity of the engine lubricants. High V.I. values have conventionally been attained by the use of V.I. improvers e.g. polyacrylates, but there is a limit to the degree of improvement which may be effected in this way. In addition, V.I. improvers tend to undergo degradation under the effects of high temperatures and high shear rates encountered in the engine, the more stressing conditions encountered in high efficiency engines result in even faster degradation of oils which employ significant amounts of V.I. improvers. Thus, there is a continuing need for automotive lubricants which are based on fluids of high viscosity index and which are stable to the high temperature, high shear rate conditions encountered in modern engines.
Synthetic lubricants produced by the polymerization of alpha olefins in the presence of certain catalysts have been shown to possess excellent V.I. values, but they are expensive to produce by conventional synthetic procedures and usually require expensive starting materials.
Another approach to the production of high VI oils has been to subject petroleum waxes to severe hydrotreatment (hydrocracking) over an amorphous lube hydrocracking catalyst, followed by dewaxing to target pour point. In processes of this type hydroisomerization of the wax takes place to form high VI iso-paraffins of low pour point. Processes of this kind are described, for instance, in British Patents Nos. 1,429,494, 1,429,291 and 1,493,620 and U.S. Pat. Nos. 3,830,273, 3,776,839, 3,794,580, and 3,682,813. In the process described in GB 1,429,494, a slack wax produced by the dewaxing of a waxy feed is subjected to hydrocracking over a bifunctional hydrocracking catalyst at hydrogen pressures of 2,000 psig of higher, followed by dewaxing of the hydrocracked product to obtain the desired pour point. Dewaxing is stated to be preferably carried out by the solvent process with recycle of the separated wax to the hydrocracking step.
In processes of this kind, the catalyst is typically a bifunctional catalyst containing a metal hydrogenation component on an amorphous acidic support. The metal component is usually a combination of base metals, with one metal selected from the iron group (Group VIII) and one metal from Group VIB of the Periodic Table, for example, nickel in combination with molybdenum or tungsten. Modifiers such as phosphorus or boron may be present, as described in GB 1,350,257, GB 1,342,499, GB 1,440,230, FR 2,123,235, FR 2,124,138 and EP 199,394. Boron may also be used as a modifier as described in GB 1,440,230. The activity of the catalyst may be increased by the use of fluorine, either by incorporation into the catalyst during its preparation in the form of a suitable fluorine compound or by in situ fluoriding during the operation of the process, as disclosed in GB 1,390,359.
A novel lower cost process for the preparation of alpha olefins useful in the production of synthetic lubricants by oligomerization with Lewis acid catalyst such as AlCl.sub.3 is described in U.S. patent applications Ser. Nos. 07/571,345 and 07/545,347, filed Aug. 23, 1990, to which reference is made for details of such processes. The process, in brief, involves the thermal cracking of slack wax to produce a mixture of alpha olefins. A portion of the mixed alpha olefins is oligomerized using Lewis acid catalyst. A high viscosity, high VI value lubricant base stock is produced by the process. Typical values for the lubricant product from oligomerization of alpha olefins from the thermal cracking of slack wax are a viscosity of above 30 cS at 100.degree. C. and a VI of about 126.
Specific automotive engine lubricant oil formulations, such as 10W-30 engine oil, have required the use of specific lubricant base stock in order to provide the requisite viscosity, lubricity, high viscosity index and other properties. In turn, the production of the specific lube base stock has been locked into certain raw materials and processes capable of producing lube stock with the requisite properties.
Low viscosity lubes can be produced from neutral slack wax by hydroisomerization. For instance, a lube basestock with a viscosity of 5-6 cS (100.degree. C.) and high VI (HVI) can be produced by hydroisomerization of HNSW in yields of 60 to 65 percent, for example, using a Pt/zeolite beta catalyst, as described in U.S. patent application Ser. No. 07/548,701. However, the production of high VI material from neutral slack wax in high yields is limited to low viscosity products with a viscosity of 6 cS or less, even if the wax is obtained from a heavy neutral oil (HNSW). Higher viscosity products with high VI, such as an 8 cS (100.degree. C.) lube stock preferred for the formulation of 10W-30 engine oil, can only be obtained from a waxy feed of higher average molecular weight, namely, a petrolatum wax (the wax from dewaxing a deasphalted residual oil such as bright stock). Consequently, petrolatum is the preferred raw material for 8 cS HVI lube stock. Petrolatum is, however, a feedstock of limit availability and there is the additional difficulty that the yield of 8 cS HVI product from petrolatum is low, about 36%.